The present invention relates to a line receiver circuit for receiving differential signals from a symmetrical transmission line. A line receiver circuit of this kind is known from WO 95 17763.
A line receiver circuit serves to receive signals from a transmission line. Differential signal transmission over a symmetrical transmission line means that the signals to be received at an input of the line receiver do not necessarily use a well defined potential of the line receiver circuit, e.g. its ground potential, as a reference but use a potential of a second line receiver input as a reference.
The common mode voltage on the transmission line, as seen by the line receiver, can not always be confined to a well defined voltage or limited voltage interval. This has numerous reasons. For instance, different system components interconnected by a symmetrical transmission line, can have an offset in their respective ground reference potentials. Another reason can be, that the line receiver circuit must fulfill the design objective to cooperate with a number of different signal transmission standards each providing a different common mode voltage level.
In reality, however, a line receiver circuit will not be able to operate with common mode voltages over an arbitrary large range. If the common mode voltage on the transmission line exceeds the limits of the common mode voltage range of the line receiver, the reception of signals from the transmission line will become unreliable or impossible. Therefore it is desirable to provide a line receiver circuit with a common mode input voltage range as large as possible.
In order to achieve this, from WO 95 17763 it is known to provide two input stages in parallel, which are designed to operate within different, partially overlapping common mode voltage ranges. If the common mode voltage at the input of that circuit reaches a predetermined level where the second input circuit can operate, the first input circuit is disabled in order to avoid that both input stages simultaneously drive a subsequent stage, because this might result in an undesirable dependency of the overall signal delay on the input common mode voltage level. The circuit according to this document achieves a takeover between said first and said second stage in a lower region of the overall common mode voltage operating range.
It is the object of the present invention, to provide a line receiver circuit with a large common mode operating range such that a take over between said first stage and said second stage takes place in a higher region of the overall common mode operating range.